In U.S. Pat. No. 4,199,663, Autonomous Terminal Data Communications System, Herzog, issued Apr. 22, 1980 to the assignee of the present invention, a data communications system is described which permits each of a plurality of terminals to periodically transmit messages on a data communications medium such as a data bus, with the message transmissions by each terminal being autonomous with respect to the message transmissions by any other terminal. A message transmission by a terminal is permitted only when a certain protocol is satisfied. The protocol requires, inter alia, that there be an absence of messages on the data communications medium for a time substantially equal to that of an intermessage or "terminal" gap unique to the terminal, and, that a predetermined transmission interval, from a previous message transmission by the terminal, have elapsed. The transmission intervals of all terminals each have substantially the same duration which is greater than the sum of all terminal gaps and of the durations of all messages transmitted by the plurality of terminals since a previous message transmission by any terminal.
Although such a system is advantageous in permitting each participating terminal to have autonomous access for the purpose of message transmissions to the data communications medium so that a central bus controller to regulate message transmissions is not required, in permitting a significant reduction to be made in the wiring requirements between participating terminals, and in permitting a significant increase in the reliability of the data communications system, the requirement for periodic message transmissions by a terminal is disadvantageous in some respects. For example, once having transmitted a message on the data communications medium, a terminal must wait until its transmission interval elapses before initiating another message transmission. If the data communications system is designed for a large number of participating terminals and only a few of the participating terminals are engaged in message transmissions, it will be seen that the requirement for the elapse of a fixed transmission interval limits the rate at which any terminal can successively engage in message transmissions and also results in the data communications medium being silent and therefore under-utilized for a significant portion of time.
Another requirement of the protocol in U.S. Pat. No. 4,199,663 is that successive messages from any terminal must be substantially equal in duration in order to maintain the periodicity of message transmissions by that terminal. Accordingly, if a large amount of data is present at one terminal for transmission to other terminals, the requirement for a fixed message duration may well dictate that only a portion of that data be transmitted at one given time and that succeeding portions of that data be transmitted upon the elapse of successive transmission intervals.
In certain applications of the data communications system in modern aircraft, the requirement for periodic transmissions of substantially fixed-duration messages is ideally matched to the requirements of the avionic system using the data communications system. For example, the avionic system may comprise a closed-loop servo system in which a transmitting terminal transmits data obtained from a sensor and in which a utilization device associated with a receiving terminal acts upon the data transmitted by the transmitting terminal to effect certain control functions. In such an application, periodic message transmissions are required so that the control actions effected by the utilization device have a predetermined time relationship to the data from the sensor. However, there are certain other aircraft systems, such as a monitoring and display system, in which the requirement for periodic message transmissions may result in degradation of system operation. In a typical monitoring and display system, certain subsystems associated with a plurality of transmitting terminals each accumulate a large block of data which should be transmitted as a unit to a receiving terminal associated with a utilization device providing centralized monitoring and display functions. Further, the time period of accumulation of data by any such subsystem, and the amount of data in any such block, may be variable. Therefore, if the data communications system is capable of permitting any terminal to engage only in periodic transmissions of substantially fixed-duration messages, and if one of the subsystems is ready to transfer a variable block of data at a time when other subsystems are not ready for data transmission, it will be seen that the block of data from the "data ready" subsystem may not be transmitted as a unit, that the rate of data transmission may be limited, and that the data communications medium may be silent for a substantial period of time.
It is therefore an object of this invention to provide an improved autonomous terminal data communications system.
It is a further object of this invention to provide such a communications system which permits each terminal to engage in aperiodic message transmissions on the data communications medium.
It is yet a further object of this invention to provide such a data communications system which also permits each terminal to engage in variable-length message transmissions on the data communications medium.
It is still a further object of this invention to provide such a data communications system which also permits each terminal to engage in periodic data transmissions on the data communications medium.
It is another object of this invention to provide such a data communications system which permits each terminal to engage in either aperiodic or periodic message transmissions on the data communications medium, depending on the condition of message transmissions by other terminals in the system.
It is yet another object of this invention to provide such a data communications system which provides optimum utilization of the data communications medium under all conditions of message transmissions by the participating terminals of the system.